Electric furnaces have been used since the nineteenth century to melt refractory or reactive materials. They are now the most common means for melting practically all ceramics and all high-melting point materials that react readily with air. The type of electric furnace most favored for melting large quantities of material is the electric carbon arc furnace with carbon electrodes as shown in FIGS. 1a and 1b. The carbon arc furnace device 10 is simple to construct and operate, but suffers from the disadvantage that its carbon electrodes are in close proximity to the product 13. This proximity of the electrodes to slag spray near the surface of the melting material 13 and their high temperature oxidation typically results in excessive electrode erosion, thus increasing the cost of the operation and sometimes contaminating the product. For example, the cost of electrode erosion during manufacture of zirconia from zircon may be more than 15% of the total manufacturing cost.
The conventional plasma arc furnace 15, shown in FIG. 1c, is an improvement over the carbon arc furnace in some respects. An arc is struck between a non-consumable electrode 16 inside a plasma torch 17 and the charge as before, but a protective flow of gas 18 past the electrode in the torch avoids the erosion problems of the carbon arc furnace. The current in the furnace usually runs through the charge to a counter-electrode 19 at the bottom. In other respects it is similar to the carbon arc furnace.
Both the carbon arc furnace and the conventional plasma arc furnace do not have a favorable geometry for heat retention in the melt. As can be seen from FIGS. 1 a-c the molten region is wide and flat to prevent the passage of current directly across the surface to a sidewall. This geometry leads to considerable heat loss; for materials with melting points over about 2000K, radiation is the main source of heat loss, so the upper surface will lose especially large amounts of heat to result in an unfavorable ratio between the initial charge of material and the amount of melt obtained from it.
A second disadvantage of the conventional furnaces is the large amount of solid material relative to the amount of molten material in them. The walls of these furnaces cannot be brought close to the arc heat source, since then electric arcs might strike directly to the walls.
A third problem associated with the carbon arc furnace and the conventional plasma arc furnace relates to discharge of the molten product. For casting processes not involving very refractory or reactive materials, it is best to discharge the product on a continuous basis. This limits the size, and hence the cost, of the molds and handling equipment and eases the task of making material of consistent quality. Such continuous casting has not been possible (except on operations of uncommonly large scale) with carbon arc or conventional plasma furnaces melting refractory materials because of the tendency of the product to freeze in the spout from which the pouring is done.